This invention relates to inverters for electroluminescent (EL) lamps and, in particular, to an inverter which can be used with an EL lamp having one electrode grounded.
An EL lamp is essentially a capacitor having a dielectric layer including a phosphor powder which glows in the presence of a strong electric field and a very low current. The dielectric layer is held between two electrodes, one of which is transparent. Because the EL lamp is a capacitor, an alternating current (AC) must be applied to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage and the current through the EL lamp ceases.
For wristwatches and other applications such as pocket pagers, an EL lamp is driven by an inverter which converts the direct current from a small battery into alternating current. The battery voltage, typically one to three volts, limits the voltage which can be applied to a lamp by the inverter. In order for the EL lamp to glow sufficiently, a peak-to-peak voltage in excess of one hundred and twenty volts is necessary. Converting from three volts to one hundred and twenty volts is difficult without a transformer and a transformer is too bulky and expensive for a wristwatch and for many other applications.
To increase the voltage across an EL lamp, the prior art teaches connecting the EL lamp across the AC diagonal of a bridge circuit and connecting the output from an inverter across the DC diagonal of the bridge circuit. The bridge circuit electrically reverses the connections between the EL lamp and the inverter, reversing the polarity of the applied voltage. The result is an approximate doubling of the voltage across the EL lamp and an alternating current through the lamp.
One technique for converting low voltage to high voltage uses a pump circuit in which energy is stored in an inductor. Typically, an inductor and switch transistor are connected in series across a battery. The junction of the switch transistor and the inductor is connected to one terminal on the DC diagonal of the bridge circuit. When the transistor shuts off, the collapsing field in the inductor generates a high voltage pulse. A series of such pulses charges the lamp and the lamp glows. The polarity of the applied pulses is reversed by the bridge circuit and the lamp then charges to about the same voltage but the opposite polarity.
The frequency of an alternating current through an EL lamp affects the life of the EL lamp, with frequencies below 1000 hz. being preferred. Too low of a frequency causes a noticeable flicker and low brightness. Thus, a frequency of 100-1000 hz. is preferred. Since the inductor and the lamp form a series resonant circuit, a large inductor is required for resonance at low frequencies. To overcome this problem, a high frequency (10-100 khz.) pulse train is combined with a low frequency pulse train and applied to the base of the switch transistor. By using bursts of high frequency pulses, the inductor can be made significantly smaller. If a bridge circuit is used, only the low frequency pulses are applied to the control electrodes of the switching devices in the bridge circuit.
A problem with bridge circuits is that the EL lamp must be electrically isolated in order to be able to connect the lamp to the AC diagonal of the bridge circuit. For many applications, e.g. watch faces, the lamp is nearly as wide as the package for the device, making isolation difficult. Being able to ground one electrode of the lamp simplifies construction of the device containing the lamp. More importantly, being able to ground one electrode enables one to drive a multi-segment display from a single inverter by individually grounding each segment through a series transistor. U.S. Pat. No. 3,908,150 discloses a multi-segment display in which each segment is separately grounded through a transistor.
Another problem with bridge circuits is stability, particularly bridge circuits using SCRs instead of transistors. The high voltage pulses from the inverter are coupled to the control electrodes of the transistors or SCRs by the parasitic capacitance inherent in the device, causing erratic triggering. Occasionally, an EL lamp is discharged when it is supposed to be charged and vice-versa. Stability becomes more of a problem as the input voltage to the bridge circuit is increased.
In view of the foregoing, it is therefore an object of the invention to provide an inverter which can be used with an EL lamp having one electrode grounded.
Another object of the invention is to provide an inverter having a single high voltage terminal for connection to an EL lamp.
A further object of the invention is to provide an inverter having sufficient voltage to drive an EL lamp without using a bridge circuit to reverse the connections to the lamp.
Another object of the invention is to provide an inverter for EL lamps in which the inverter includes an inductor having two terminals that are alternately connected to a single high voltage terminal of the inverter.
A further object of the invention is to provide an inverter in which power consumption is reduced by recycling the energy within the inverter.